Apparatus for generating sound waves



M y 22, 1945. E. H. LAND ET AL- 2,376,493

APPARATUS FOR GENERATING SOUND WAVES Filed Sept. 16, 1940 3 Sheets-Sheet 1 BY M I O EY, I

ATT-

' INVENTO y 1945- E. H. LAND ET AL 2,3

APPARATUS FOR GENERATING SOUND WAVES .Fil ed Sepc. 16, 1940 3 Sheets-Sheet 2 ATTORNEY May 22, 1945 E. H. LAND ETAL APPARATUS FOR GENERATING SOUND WAVES Filed Sept. 16, 1940 s Sheets-Sheet s INVENTOR Patente'd May 22, 1945 FICE APPARATUS FOR GENERATING some WAVES Edwin H. Land and Martin Grabau, Cambridge,

Mass,

assignors to Polaroid .ilorporation,

Dover, Del, a corporation of Delaware Application September 16, will, Serial No. 35?,t3d as illaims. lei. si-iis) This invention relates to a new and improved apparatus and to an improved process. for genersting sound waves.

An object or the invention is to provide a method and apparatus employing polarized light for the generation of superaudible, audible or sub-audible sound waves.

A iurther object of the invention is to provide a simple, emcient, inexpensive and readily manuiactured mechanism fr"- accomplishing the desired result.

Further objects of the invention are to provide a method and means for setting up fluctuations in the intensityof a beam of light, and more specifically fluctuations of a sinusoidal nature; to provide a means and method for translating these fluctuations into sound waves; to provide 7 means in connection with apparatus of the character described for altering the fundamental frequencies of the sound waves generated; to

provide means for the simultaneous production of a plurality of sound waves of different fundamental frequencies; to provide means for the production of a sound wave or sound waves of predetermined fundamental frequency or irequencies with the natural harmonics thereof; to provide means for controlling the relative strengths of the said harmonics or overtones and the fundamental frequencies; and to provide means for altering the fundamental frequency of a sound wave produced.

Still further objects of th invention are to provide a plurality of relatively rotatable elements for mechanically and periodically altering the intensity of a beam of light; to provide means associated therewith for translating said alterations into sound waves; to provide means for modifying. said periodic alterations without altera ing the relative speed of rotation of said elements;

to provide photoelectric and translating meansadapted to receive and reproduce the fluctuations introduced into a plurality of light beamslnseparate optical systems; and to provide means assointensity or the light source or sources employed.

Still further objects of the invention are to provide, in a device of the character described, a plurality of relatively rotatable light-polarizing elements interposed in the path of a beam of light for eliecting fluctuations in the intensity thereof, and to provide in connection therewith a plurality of light-transmitting elements, such for example as doubly retracting elements or light-polarizing elements, or fractional wave plates for efiecting predetermined desired alterations in said fluctuations.

Other ob'jectsof the invention are to provide a musical instrument of the character described adapted to produce any desired tone or combination of tones of any desired quality and with any desired combination of natural harmonics; and to provide a device oi the character described for use in connection, for example, with signaling devices, such as fog signals and the like, and adapted to produce a tone or tones of predetermined fundamental frequency and quality.

The invention accordingly comprises the several steps and the relation of one or more. of such steps with respect to each of the others, and the apparatus embodying features of construction, combination oi. elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the accompanying drawings, in which:

Figure l is a diagrammatic representation of a simple apparatus embodying one form of the invention; f

Fig. 2 is a similar view of a modified form of the invention;

Figs. 3-6 are similar representations of additional modifications of the invention;

. Fig. 7 is a diagrammatic representation of a modified form of the invention wherein a plurality of difierent sets of related elements are posed in the of a beam of light and where one of said elements is rotated and one other held in fixed position, the variation 01' the operative relation of these elements will produce a fluctuation in intensity in the transmitted beam, the beam fluctuating from maximum to minimum intensity with twice the frequency of the speed of rotation of the rotated element, i'or example, if the rotated element is rotated at a speed or 220 revolutions per second, the transmitted beam will fluctuate at a frequency of 440 vibrations per second. It the fluctuations of the intensity of the beam are translated into electrical oscillations of the same frequency without distortion and these in turn are translated into mechanical vibrations of a suitable diaphragm in a loud speaker, again without distortion, a tone will be produced. This tone will be a pure tone substantially without upper harmonics or,overtones of a frequency of 440 vibrations per second, i. e., it will be an audible tone and will have that pitch produced by striking A above middle C on a piano. Fig. 1 of the drawings illustrates a simple form of a device adapted to produce such a result.

In Fig. 1, element represents any suitable light source, preferably adjacent a. reflector l2, and elements I4 and I6 represent light-polarizing elements. Element I6 is represented as stationary and fixed in position, and element I4 is mounted to rotate as in the direction indicated by arrow it about shaft 22, which may be driven by any suitable source, for example the motor !5, and at any desired speed.

A photoelectric cell or similar device 20 is positioned in the path of a beam oflight'traversing elements It and IQ. Said cell should be such that the range of wavelengths to which it responds should include at least a portion of the wave length range which the polarizing elements are adapted to polarize. An amplifier 24 is represented in circuit with the photoelectric cell, and said amplifier in turn has a translator or loud speaker 25 in circuit therewith. It should be pointed out that, although a photoelectric cell or tube is preferred, they may alternatively be used any similar device which is responsiveto Variations in intensity or the plane of polarization of incident light, for example a thermo-couple. Moreover, it should be understood that although the invention is shown and described in connection with an amplifier and loud speaker, any translating means, such as that indicated, or any recording means, such as a moving photographic plate, may be used, 'if it is responsive to activation of the and claims herein.

Suitable light-polarizing elements for use in apparatus embodying the present invention may.

comprise discs or other like elementsof any suitable light-polarizing material, for example, the materials made and sold under the trade name Polaroid. If desired, these materials, which are available in thin sheets of adequate tensile strength for the purposes of this invention, may be employed in unmounted or unlaminated form, or it may be mounted upon or laminated to a plate or between plates 01' glass, plastic or other light-transmitting material. It is not essential that the polarizing material employed function to etlect complete polarization of the transmitted beam.

Fluctuations in the intensity of the transmitted beam arise from the relative rotation of the element I4 with respect to the element 88 and give rise to corresponding oscillations in the current delivered by the photoelectric cell 20. These oscillations, after being amplified as at 24, are translated into sound waves by the translator 25 and, if the frequency of rotation of the element 14 is such as to give rise to audible frequency sound waves at the translator device, an audible tone is produced, Alterations in the speed of retation of the element M, which may he accomplished by any control at the driving element or in any other desired manner, give rise to corresponding alterations in the pitch of the tone produced at the translator. Conventional power control means 50 are suitably connected to the driving element l5 whereby the latter may be regulated to alter the speed of rotation of the light-polarizing element l 4 as desired. With the device shown in Fig. 1, therefore, it is possible to produce a variety of difierent, pure tones of any desired pitch and of any desired intensity within the physical limits of the apparatus.

It appears desirable in connection with the production of tones of high pitch in the audible range and of supersonic sound to employ a device such as shown for example in. Fig. 2,. or obvious modifications thereof, so as to avoid excessively high speeds of rotation, as will hereafter he described. It'is possible with other modifications of the device to produce overtones or harmonics of the basic pure tone without altering the speed oi rotation of the elements and it is also possible to control the relative intensity of these overtones with respect to the basic tone in such men ner as even to eliminate the basic tone as well as undesired overtones, each of which is a tone haying a frequency which is an integral multiple of the basic rotation frequency, and to produce'oniy the desired tone. Thus, for example, the basic frequency of rotation is 220 revolutions per second, an isolated overtone may readily be produced having a frequency of 2640 vibrations per second or even a greater or lesser frequency if desired.

It is thus apparent that even with relatively low speeds of rotations, relatively high pitched tones may be produced. This is particularly desirable in producing supersonic or super-audible tones as, for example, for use in beacons or signaling.

In Fig. 2, both of the discs of the polarizer, here designated as 2 l4 and 2i 6, are shown mounted to rotate on shafts 222 geared together by gears 226 and positioned in the path of the beam emanating from the light source it and impinging upon the photoelectric cell 2c, As shown, polarizing element 2 is adapted to rotate. in'a counterclockwise direction and polarizer tilt in a clockwise direction. This rotation. oi the two elements quadrupies the frequency of the fluctuation set up in the transmitted beam over the frequency of rotation of either shaft and thus permits the production, in the more complicated forms of the invention employing this arrangement, of very high pitched tones without subjecting the elements of the mechanism to excessive speeds of rotation. As in the modification shown in Fig. 1, the frequency of the fluctuations in the intensity. of the transmitted beam may be varied by varying the speed of rotation of the light-po larizing means. For this purpose use is made of conventional power control means 256 which are designed to permit regulation of the driving element 15 and hence the rotative speed ofthe light-polarizing elements 2 I4 and HQ.

Another method of accomplishing the same result, and by the use of rotation elements mounted on a single shaft, is to add to the device tone with its second and third harmonics.

l shown in Fig. 1 a second, rotatable, polarizing l element positioned between fixed polarizer 16 and polarizing axis at right angles to the polarizing axis of the other, the fluctuations of intensity created in the transmitted beam have four times the frequency of rotation of the polarizers. If

'now the phase relation of the two rotatable polarizing elements is changed, i. e., if the relative orientation of their polarizing axes is altered from that described toward parallelism, there appears with such alteration a sub-harmonic tone, and the relative intensity of this sub-harmonic increases with the change of phase relation until the polarizing axes of the elements are parallel, at which position the intensity of the sub-harmonic is V a maximum. This sub-harmonic has one-half the frequency of the tone that was produced when the two rotating polarizing elements were set with their polarizing axes perpendicular and with the fixed polarizer between them.

y In Fig.3 there is shown a modification of the device shown in Fig. 1. This device comprises two rotatable polarizing elements 3!!! and 3M.

and two fixed polarizing elements 316 and. Hi, all positioned in the path of a beam emanating from a light source I and impingingupon photothat of the frequency of rotation of the shaft and a still weaker component having a frequency six times the frequency of rotation of the shaft, i. e., the tone produced comprises a fundamental If the relative orientations of the polarizing elements are in any way altered, a change in the relative intensity of the fundamental and its harmonics is produced. In particu1ar, if the orientation of the polarizing axis of the element 3M is 60? from the orientation of the polarizing axis of the element 3| 3 in one sense, i.e., either clockwise or counter-clockwise, and. if the orientation of the polarizing axis of the fixed element ill is 60 from the orientation of the polarizing axis of the element 3l6 in the opposite sense, then the fundamental frequency, i. e., first harmonic,-

and the second harmonic are substantially suppressed and the tone produced comprises primarily the third harmonic of the original fundamental. Alteration of the speed of rotation of the shaft which carries the rotatable light-polarizing elements (H3 and 3|4 will also alter the frequency of the fluctuations in the intensity of the light incident on the photoelectric cell 20. To this end use is again made of power control means, indicated by the reference numeral 350 in Fig. 3 for the purpose of regulating the driving element 15 and altering the rotative speed of the lightpolarizing elements 3l3 and 3 at will.

Other combinations of rotatable and fixed elements in various relative orientations maybe employed to produce a wide variety of combinations of fundamentals and their respective upper harmonies with varying intensities of upper harmonics with respect to each other and with respectto the fundamental harmonic. Any funda- 1 vention.

It is thus 'obvious that, by a proper selection of the number of rotating and fixed polarizers and by a proper arrangement of the phase or orientation relationships of successive polarizers with respect to each other, it is possible to isolate the highest harmonic of the fundamental tone of a particular combination and to suppress the fundamental and all the harmonics except the highest. Similar modifications of the apparatus shown in connection withthe device illustrated diagrammatically in Fig. 2 will be obvious, and are deemed to fall within the scope of the invention.

The apparatus has thus far been described as one employing fixed and rotatable light-polarizing elements as the means for causing predetermined fluctuations in the intensity of a transmitted beam. In Figs. 1, 2 and 3 the rotatable elements are shown mounted on a shaft. It will be obvious that they may be mounted in other ways. In Fig. 6, for example, the rotatable elements 5M are shown mounted at each side of fixed polarizer 6m to rotate on bearings" 62!, and

are shown as being driven by a shaft 522 provided with gears 626 adapted to engage suitable gears 628 on the peripheries of the polarizing elements. Power control means 650, similar to those shown, are suitably connected to the driving element l5 of Fig. 6 whereby regulation of the rotative speed of the light-polarizing ele= ments SM is effected. With such a system the entire area of the polarizers may be employed to intercept the transmitted beam. It will be obvious that othermethods of mounting and'rotating the rotatable elements and of mounting the fixed elements may be employed, and that all or any desired portion of the surface area of any of the elements may be employed to intercept the transmtted beam.- In this connection it might be pointed out that the intensity of the tone produced may be controlled in part by con trolling the quantity of light falling upon the photosensitive element, i. e., as theintensity or cross-sectional area of the light beam is increased, the intensity of the tone produced by the system may be increased. Control means of this element adjacent the light source and the polarizing element adjacent the photosensitive element, and these birefringent or half-wave retardation devices. may be employed as rotating or as fixed, I

or as fixed and rotating elements in apparatus embodying the invention. All of these elements,

including the light-polarizing elements andthe birefringent elements are termed herein optically anisotropic elements. In, for example, the apparatus shown diagrammatically in Fig. 4, the two polarizing elements 6 are fixed with respect to each other, and have between them a halfwave retardation device 430 mounted for rotation with shaft 22. The rotation of the half-wave device will produce variation of the operative relation of the polarizing elements, and the resulting fluctuations in the light beam traversing the three elements give rise in the translator to atone having a frequency four times the frequency of the speed of rotation of the half-wave device. So also, if element H6 in Fig. 6 represents a half-wave device fixedly mounted between the two rotating elements 6, the same result is obtained. Variations in the fluctuations in the intensity of the transmitted beam may also be attained in the device of Fig. 4, by power control means 450 associated with the driving element l5 for regulating the speedof rotation'of thehalfwave retardation means 430. A further modification is shown in Fig. 5, an

comprises a fixed polarizing element SIG, a rotating polarizing element 5, a fixed half-wave plate 530 adjacent the rotating polarizer but between it and the fixed polarizer, and a rotating half-wave plate 53! mounted on shaft 22 for rotating with the rotating polarizer and positioned between the fixed half -wave plate and the fixed polarizer. With this arrangement a pure tone will be produced in translator 25 having a frequency six times the frequency of the shaft speed, and other modifications of added half-wave devices fixed and rotating to give tones of higher pitch may also be used and will he understood to fall within the scope of the invention. Power control means-550 are also employed in the device of Fig. 5 to the end of regulating the frequency of thefiuctuations in the intensity of the transmitted light by regulating the r'otative speed of the rotatable members 5M and 532.

Another arrangement involving the use of half wave retardation devices may be produced by, modifying the device shown, for example, in Fig. 2, where two polarizing elements areemployed, one adjacent the light source, the other adjacent the photocell, each mounted for rotation on a separate shaft in a direction opposite the direction of rotation of the other. If this apparatus is modified by mounting with each rotatable polarizer a'half-wave plate, so that the structure comprises, successively, and in 7 addition 'tQQthe I light source, the photocell and the translator, a polarizing element mounted for clockwise rotation, a half-wave element mounted for counterclockwise rotation, a second half-wave. plate mountedfor clockwise rotation. and a second polarizer mounted for counter-clockwise rotation, and if the beam impinging upon the photoelectric cell traverses all of said rotating elements in the order mentioned, the tone produced will be a pure tone having a frequencytwelve times the frequency of the speed of rotation of the shafts, and the characteristics of the tone will remain unchanged irrespective of changes in the relative are fixedly positioned with respect to each other,

the resulting tone produced will have a frequency eight times the frequency of the shaft rotation. As the number of relatively rotatable half-wave devices between the polarizers in structures such as are shown mm. 2, Le, where two shafts are In this connecticn it should he pointed out that, strictly speaking, a retardation plate can bee, so-called half-wave plate only for one wavev length of light. For neighboring wavelengths of tion is 0.69 wavelength at 4009 A. and 0.39 wave (I light, it will not he a perfect half-wave plate. If white light (comprising the usual distribution of light energy in the spectral range from 4060 to 7000 A.) is used, and if the half-wave plate is designed for the wavelength, say 5500 A. where the sensitivity of the eye is greatest, then its retardalength at 7000 A. Hence, in any system in which a combination of fixed and rotating polarizing sheets and half-Wave plates is designed to produce a pure tone, the tone will'still involve traces of harmonics whose relative intensity can be calculated from the spectral distribution of intensity of the source, the spectral distribution of retards.- tion of the half-wave plate or plates, and the spectral distribution of response of the photoelectric cell.

This deficiency can easily be corrected by the use of monochromatic light, such as an electric sodium lamp, whose light energy is concentrated in two intense yellow lines in the spectrum which are very close together (58% and 5&96 A.) and designing the half-wave plates for this narrow region of the spectrum. Another method involves the use of a common mercury vaporiamp, or a neon lamp whose spectral distribution of intensity lies almost entirely in the region icetween 6000 and 2000 A. and designing the halfwave plate accordingly.

In order to out out far rec. and infra red light, for which Polaroid film is less perfect as a, polarizer than elsewhere in the visible spectrum and where the sensitivity of a. photocell is large, a

suitable filter may he interposed in'the system;

such as a Zeiss BG -is. It should he pointed out, however, that this is not as important a. problem as it seems, for the reason that it is only the variation in the polarizedradiation which causes the system to operate.- In other words, it is the light fluctuations caused by the polarizers which give rise to the tones produced, and light reaching the photocell without interception by two or more polarizers will be constant and will have no eflect thereon except to establish its minimum illumination.

Other modifications of shaft and element arrangements than those previously discussed are shown in Figs; 7 and 8. In Fig. 7, for example, three tone-producing units, comprising a pinrality of overlapping polarizing elements H5 and I portion of the beam through the predetermined elements which comprise each unit, and then to direct each of said portions so that it impinges upon the photo cell. Supplementar directional mirrors 1 may be'employed, if desired, and shutter means, indicated at. 145, may be employed to block any of the beamsemanating from light source "0. With such a device a plurality of separate tones including, if desired, their overtones, may be produced with the use of a single light source and a single rotating shaft. As in the other modifications of the invention, power control means [50 are employed in Fig. '7 to regulate a driving means or element 1 l5 which drives the shaft I22 whereby to vary at will the rotative speed of the shaft and the'light-polarizing elements H4 carried thereby to the end of predeterminedly alteringthe fluctuations in the intensity of the light transmitted to the photoelectric cell I20.

In Fig. 8 a modified structure is shown wherein a plurality of light sources Blll are provided, each with its own rotating shaft unit 822. This unit may comprise in any case a predetermined arrangement of fixed and rotatable elements, including polarizing elements and half-wave devices. The shafts may, if desired, be separately driven by separate power means or driving elements M5, M5 and M5"; or they may be so mounted as .to be driven from asingle power source by means of separate gear trains, not shown, whereby different'shaft speed may be secured. Variation in the rotative speeds of the different shafts 822 may be obtained through individual, power control means 850, 85l and 852 suitably connected to the driving elements BIS,

' M5 and M5" respectively. The beams traversing the elements associated with each shaft may be brought to focus on a photoelectric cell 820 mounted at the'focal point of a parabolic refiector M2, and the tone may be produced in the translator and amplifier 825. Here, again, control of the tone production may be secured by controlling the energization of the light sources separately. For this purpose, use is made of separate control means 880, Bil and 8-52 of conventional design, each connected to-an individual light'source 8l0 whereby the intensity of the source may be regulated as desired. As previously intimated, if desired intensity .cont'rol means of this character may be employed with the light source of the other embodiments of the invention for controlling the intensity of the light structure shown in Fig. 8 may readily be modified by arranging the light sources and their associ ated shaft units radially about the photoelectric I ous; In any of the modifications of the invention it will be understood that .means may be provided to bring predetermined elements into engagement with other'elements or similarly to remove them from such operative engagement. For example, the. half-wave retardation devices mount-f they may be swung out, of operative relationship with respect to the other elements. For exaI.-

ple, as shown in Fig. 9, an element 9M may be fixed to a rotatable shaft 900 for rotation therewith. Element 90| may be a light-polarizing device or fractional wave retardation means. It is shown in operating position in the path 902 of a light beam adapted to be transmitted onto a photoelectric cell of any of the embodiments of the invention. By rotating shaft 900 in the counterclockwise direction of the arrow at the left end of the shaft, element 9M may be swung out of the path 902 of the beam. counterclockwise rotation of the shaft will then return element 9M to its operating position shown in Fig. 9. All such methods of controlling the pitch or tone qualities of the sound produced by devices embodying the present invention are to be deemed within the scope thereof. It may also be pointed out that by the use of a number of the above described units, suitably mounted and arranged and provided with appropriate controls, a musical instrument may be produced which willv have the general characteristics of an organ. Similarly,

' conventional musical instrument.

It should be understood that it is also practicable to arrange the variouspolarizing and halfwave elements so thatcertain thereof intercept only a portion of the light beam. Thus, a portion of the beam may be intercepted at one frequency and the remainder at another frequency,

which will give rise to a still more complex combination of overtones. In a still further modification, elements rotating in the sam'edirection may be caused,.as by suitable gearing, to rotate at different relative speeds, thus making it possible, for example, to secure extremely low frequencies with a minimum of -gearing and at any desired shaft speed.

- emitted bythem. It will be-obvious that the Taroouousrarc EXPLANATION or THE Invrzn'rron It is believed that the present invention will be I understood more easily if it be explained also with respect to certain mathematical formulae involved. If it 'is assumed thatthe emergent beam from a sheet of polarizing material is completely polarized and if Jc'is the coeflicient of transmissivity of a sheet of unit thickness, then for the light which is transmitted from an incident, nonpolarized beam'of intensity I,.a unit sheet has an emergent beamof .intensity k1. If the incident beam is polarized and impinges on the sheet with 'its direction of vibration parallel to the polarizing axis of the sheet, then the emergent beam intensity is 2kI. In the following, it

will be assumed either that the light used is ed between the polarizing elements in certain of the modifications of the invention described, or certain ofthe polarizing elements in other modifications of .the invention described, "may be mounted to be swung into and out of operative relation. to the rest of the polarizing vor birefringcnt elements, whereby the tone produced ma be altered in pitch or-, in quality, or in both ways. This may be accomplished by mounting separate elements-independently on pivoted arms, so that monochromatic or that k is the same for all wave-lengths and that the polarizer completely polarizes'the transmitted beam.

- Combinations including fixed and rotating mlalrizers I If a beam of non-polarized light of intensity In passes successively through two sheets of polar izing material, the polarizing axes of which are at an angle 0, the emergent intensity I is I=kIo (2k cos 01%. (1)

or the relative intensity J i. e.,' th 'ratio 1/10, is-

If the sheet which the light beam first strikes is fixed, and the second rotates so that its polarizing axis rotates about the beam, then the direction of the electric force in the emergent beam is always parallel to the rotating polarizing axis. The intensity of the emergent light is 'proportional to the square of the electric force, and the intensity varies, in accordance with (4) with the cosine of twice the angle a. When is 90, the polarizers are crossed and the transmitted intensity is of course equal to zero. For each complete revolution of the second sheet, the intensity fluctuates through two complete cycles, each from maximum to minimum. It is to be noted, in accordance 1 with (4) that. the fluctuation is of a sinusoidal nature.

' If a fixed, third polarizing sheet is positioned to intercept the beam after it has passed through the second rotating sheet and is positioned with its polarizing axis parallel to that of the fixed, first sheet, the beam incident on the third sheet is plane-polarized, the plane of polarization rotating with and parallel to the polarizing axis of the rotating, second sheet. As just mentioned, its relative intensity J varies. When the plane of polarization of the beam incident on the third sheet is parallel with the polarizing axis of the third sheet, a maximum of the beam passes through the third sheet but, as that plane r0- tates, the amount of light transmitted is further diminished by the third sheet until said plane is perpendicular to the polarizing axis of the third sheet. The intensity of light leaving the third sheet is X=J 2!: cos 0 j (5) or substituting (2), 2lc cos a 2k cos 0 (6) :05 cos 0 (7) or substituting (3),

K=4k= cos 2a+% cos 40) (8) The fluctuating component is-made up entirely o! a component which fluctuates with a frequency which is four times that of the rotating shaft.

Similarly, when there are four polarizing sheets, the first and third fixed and the second and fourth having the same angular velocity about the light beam as an axis and all at certain identical times having their polarizing axes parallel; the ratio L of the intensity of the final emergent beamtotheorlginalincidentbeam k givenby L=8k cm 0 (11) which by similar c substitution be canes l-=8k cos 20+? cos cos 6!) (12) thefixedthirdsheettohave adirectimofpolariatimatanangleof-h/Sradians,

or -'-120, to the direction of polarization of the fixed first sheet, and considering the rotating second sheet to start from a position such that its direction of polarization is then parallel to the first sheet and considering the rotating fourth sheet to start from a position such that its direction of polarization then makes an angle of -21r/3 radians or -120, with the direction of polarization of thefirst sheet, the value of L after the rotating sheets have gone through an angle 0 is L=8Ic cos 0 cos (0+21r/3) cos (0-2103) (1 3) which becomes v L=8lc Gi l-:1 cos (14) The general rule, which includes Equations 4. 10 and 14, for determining the relative orientation of the varioussheets whereby all partial tones, except a; certain predetermined highest partial tone, are suppressed, is as follows:

Given a system of n' Polaroid discs in which alternate elements are respectively stationary and rotating, the relative intensity of light transmitted by this system is J= /2 (210" cos (e-i-ci) cos ((H-ea) cos (c-i-cn-i) (15) series will depend upon the values of the quan-' tities e1, e2, cs, etc.

In particular, however, all the harmonic components of this series, except the one of highest frequency, may be made to vanish, by a suitable choice of the values of e1, 62, ea, etc, in which and in which =21r/n-1 in radian measure =360/n-1 in degrees The expression for the relative intensity of the light is then =-;-(2l cos 0 cos (ii-F 5) cos, (mlcos (6+ (16) and this by a suitable sequence of trigonometric substitutions reduces to one in which a term in cos=(n-1)0 is the only fluctuating component.

In order to determine the initial orientations of the polarizing axes of successive Polaroid discs in the assembly, insert the condition 0:0 in the above equation for J. Assume that the oddmnnberedelements in the assembly are stationmy and that the even-numbered elements are mounted so as to be capable of a rotation in an arbitrarily chosen positive sense. It follows that the first two Polaroid discs must be so set that their polarizing axes are initially parallel. Since the second Polaroid disc is rotating positively.

In which case right angles to that of the other.

the polarizing axis of the third Polaroid disc must be set at an angle from the initial orientation of the polarizing axis of the second Polaroid disc. The third Polaroid disc is stationary. The polarizing axis of the fourth Polaroid disc must then be set initially at an. angle to the orientation of the polarizing axis of the third Polaroid disc, and this procedure may be continued for any given number of discs.

Equatioii 4 defines the intensity of a beam which has traversed a. fixed and rotating sheet when, after a given time, the direction of polarization of the latter has reached an angle with the direction of polarization of the former.

If both had beenrotating at the same rate but in opposite directions, the angles between the directions of polarization would have increased twice as fast and, after the same time, the lastmentioned angle would have been equal to 20.

Similarly, if certain intensity components are present with respective frequencies of fluctuation in a beam which has passed through fixed polarizing sheets and sheets rotating in one di- 7 A a: cos 204 sin 20 (23) The symbol is here used to signify is pro. portional to. The intensity J is, of course, proportional to the square of the amplitude A. On making the appropriate substitutions and simplifications, there is obtained:

or, for a half-wave plate, where cos d=1,

J *5 cos 80+l If the polarizing sheets, instead of having parallel polarizing axes as in the preceding 'case,

- zation of incident plane-polarized light. If such rection, then if the fixed sheetsare given an equal speed of rotation but in the opposite direction, those intensity components have twice as great frequencies of fluctuation.

Combinations including retardation plates- In the case of two polarizing sheets, with parallel polarizing axes and rotating with the same angular velocity, with an interposed retardation plate having an equal angular speed and opposite angular velocity, the polarized light received from the first sheet by the plate is resolved in the plate into two components which emerge from the plate differing inphase by an angle d and each having its direction of. vibration at The geometric reference systems have to be taken with axes along these directions. If a is the amplitude of the intensity of the light leaving the first sheet, then, at any instant at which the direction of polarization of the light incident on the retardation plate is inclinedvat the angle a to the :1: axis in the retardation plate, the amplitudes of the two components,'::: and 11, leaving the plate would be a:=a cos mt cos a" (19) y=a cos(mt+d)sin a. (20) where m is equal to 2w" times the frequency of the light.

If the first sheet and the plate rotate in cpposite directions, when each has turnedthrough an angle 0, then a will be vious equation, and

x=a cosv 20 cos m't y=a sin 20 cos (mt-i-d) The above values of the amplitude a: and 11 must now be resolved along thpolarizing axis of the second rotating sheet. The resultant amplitude is the sum of theircontributions. If A is the resultant amplitude, then equal to 20 in the prea plate is interposed between, and has an angular velocity equal in magnitude-and opposite in direction to two polarizing sheets, and-if the polarizing axis of the first sheet mak s an angle u to the positive side of either of the principal axes of the half-wave plate, then the plane of polari zation of the light emerging from the half-wave plate makes an angle u with the same principal axis of thehalf-wave plate toward the negative side. I-lence, the angular disparity between the .polarizing axis of the first polarizing sheet and of the plane of polarization of the light emerging from the half-wave plate is 2u. When the first I polarizing sheet has rotated positively through an angle 0 and (in the meantime) the half-wave Q Hence, the frequency of the fluctuation'of the rotation of either shaft.

I (compare Equations 26 and 2a).

plate has rotated negatively through an angle 0, the angle between their axes is 20. Consequently, the plane of polarization of the light emerging from the half-wave plate must have turned through an angle 40 from where it was originally. This rotation is negative, but both the first and second polarizing sheets have turned through the positive angle am the same time. Hence, the angle between the plane of polarization of the light emerging from the half-wave plate and the polarizing axis ofthe second polarizing sheet is 40. Neglecting .any absorption of light by the half-wave plate,

intensity of the light emerging from the second polarizing sheet is eight times the frequency of v If a light beam passes through a polarizing sheet. two half-wave plates and a second polarizing sheet. all rotating at the same rate but withalternate elements rotating in opposite directions, the intensity of the finally emergent beam .I=2]t?co s 60 -(29) For each' additional half-wave plate inserted, the argument of the cosine term is increased by 40 z The foregoing formulae represent the resul obtained in an ideal system. They may be lo es a asvasaa mined groups of said polarizing elements and to cause the resulting polarized beams to impinge upon said sensitive means.

8. In a tone producing device having a source of light, photo-sensitive means positioned in the 9. In a tone producing devicehaving a sourceof light, photo-sensitive means positioned in the path of light emanating from said source and speaker means associated with said photo-sensitive means and responsive to activation thereof, in combination, a plurality of light-polarizing elements positioned in said light path between said source and said photo-sensitive means. alternate elements in said plurality being mounted for rotation relatively to the remainder of said elements, means for causing said rotatively mounted elements to rotate, and means for regulating the frequency ,of rotation of said elements such that a superaudible sound is produced in said speaker means. m

10. In a tone producing device having a source of light, photo-sensitive means positioned in the j J means for rotating said rotatably mounted elements at predetermined speeds.

13. In a musical instrument having at least one light source, photo sensitive means receiving light emanating from said source and loud speaker means associated with said photo-sensitive means and responsive to activiatlon thereof,

in combination, a plurality of groups .of lightpolarizing elements traversed by light emanating from said source and incident on said loud speaker means, predetermined pairs of polarizing elements in predetermined ones of said groups having half-wave retardation means positioned therebetween, alternate elements in each of said groups being mounted for rotation relatively to the remainder of the elements therein, and means for rotating said rotatively mounted elements at predetermined speeds.

14. In a musical instrument having at least one source of light, photo-sensitive means positioned in the path of light emanating from saidsource and speaker means associated with said photosensitive means and responsive to variations in the output thereof, in combination, means for causing the light from said source incident on said photo-sensitive means to fluctuate at a predepath of light emanating from said source and speaker means associated with said photo-sensitive means and responsive to activation thereof,

in combination, a plurality of light-polarizing element= positioned in said light path between said source and said photo-sensitive means, alternate elements in said plurality being mounted for rotation relative/to the remainder of said elements, means for causing said rotatively mounted elements to rotate, and means for regulating the irequency of rotation of said elements such that a sub-audible sound is produced in said speaker means. a I

11. in a tone producing device having a source of light, photo-sensitive means positioned in the path of light emanating from said source and speaker means associated with said photo-sensitive means and responsive to activation thereof, in combination, a plurality of light-polarizing elementspositioned in the path of light emanating, from said source, and located between the source and said photo-sensitive means, at least one halfwave retardation device positioned between said polarizing elements, at least one 01 the elements in the assembly being mounted for rotation rela-. tively to the remainder of said elements, and

means for rotating said rotatably mounted elein the path of light emanating from said source tel-mined frequency, said means comprising a plurality of light-polarizing elementspositioned between said light source and said photo-sensitive means, means for causing predetermined ele-' ments of said light-fluctuating means to rotate relatively to the remainder of said elements and means for predeterminedly altering the frequency of such variations.

15. In a musical instrument having at least one source of light, photo-sensitive means positioned and speaker means associated with said photosensitive means and responsive to variations in the output thereof, in combination, means for causing the light from said source incident on said photo-sensitive means to fluctuate at a predetermined frequency, said means comprising a plurality oi light-polarizing elements positioned between said light source and said photo-sentitive means, means for causing predetermined elements ofsaid light-fluctuating means to rotate relatively to the. remainder of said elements, and g means for predeterminedly altering the frequency 01' such variations, said means comprising means for moving predetermined elements in saidlightfluctuating means into and out 01' the path of light from said source incident on said photosensitive means.

16. In a musical instrument having at least one source of light, photo-sensitive means positioned and speaker means associated with said "photo-v in the path of light emanating from'said'source sensitive means and responsive to variations in the output thereof, in combination, means for causing the light-from said source incident on said photo-sensitive means to fluctuate at a pre determined frequency, said means comprising a plurality oi optical anisotropic elements and includingyat least two light-polarizing elements positioned between saidlight source and-said photo-sensitive means, means for causing prede termined elements of said light-fluctuating means to rotate relatively to the remainder of said elements, and means for predeterminedly altering v the frequency of such variations.

1']. In a musical instrument having photo-electrio means sensitive to variations inincident light,

- speaker means associated with said sensitive responsive to activation thereof and meanssnd means ior projecting a plurality. of beams of light onto the said sensitive means, in combination means for selectively interrupting one or more of said beams at a predetermined frequency, said meanscomprising a plurality of optically anisotropic elements and including at least two lightpolarizing elements positioned in the path of each of said beams, and means for causing predetermined elements in any of said pluralities to rotate relatively to the other said elements therein.

18. In a musical instrument having photo-electric means sensitive to variation in the intensity of incident light, means for projecting a plurality of beams of light onto said sensitive means and speaker means associated with said sensitive means and responsive to activation thereof, in combination, a plurality of optical systems each comprising a plurality of optically anisotropic elements including at least two light-polarizing elements positioned in the path of each oi said beams, predetermined elements in .each of said systems being mounted for rotation relatively to the other said elements therein, and means for selectively causing the rotatable elements in any 01' said systems to rotate at predetermined irequeneies.

19. Means for creating a sinusoidal fluctuation in the intensity of a beam of light comprising a plurality of light-polarizing elements positioned to intercept said beam, and means for causing relative rotation of th transmission axes of said polarizing elements with respect to each other.

20. In combination, a light source, means for eiiecting a sinusoidal fluctuation in the intensity of light emanating from said source and comprising a plurality of light-polarizing elements,

means to position said elements so that a portion at least of each of said elements intercepts the same predetermined portion of the light emanating from said source, and means to cause relative rotation of the transmission axes of said polarizing elements with respect to each other.

21. In combination, a light source, means for eflecting a sinusoidal fluctuation in the intensity of light emanating from said sourceand comprising a plurality oi. light-polarizing elements, means to position said elements so that a portion at least of each ofsaid elements intercepts the same predetermined portion of the light emanating from said source, means to cause relative rotation of the transmission axes of said polarizing elements with respect to each other, and

means for altering the frequency of the said fluctuation in intensity of said beam. u

22. In-combination, a light source, means associated therewith for substantially collimating a beam of light emanatingirom said source, a plurality oi light-polarizing elements positioned to intercept-said cbllimated' beam, and means for rotating at least one of said elements with respect to another of said elements to effect a sinusoidal fluctuation in the intensity of light transto another of said elements to efle'ct a sinusoidal fluctuationin the intensity of light transmitted by said elements, and means for altering the speed of rotation of said rotating element to effect a, change in frequency in said fluctuation.

gara es 24. In combination, a light. source, means associated therewith for substantially collimating a beam of light emanating from said source, a plurality of light-polarizing elements positioned to intercept said collimated beam, means for rotating at least one of said elements with respect to another of said elements to effect a. sinusoidal fluctuation in the intensity of light transmitted by said elements, and means for effecting a change in the-frequency of said fluctuation.

25. In combination, a light source, means associated therewith for substantially collimating a beam of light emanating from said source, and means for eifecting a sinusoidal fluctuation in the intensity of said light beam, said last named means comprising, in combination, a multiplicity of light-polarizing elements positioned to intercept said beam, alternate elements in said multiplicity being mounted for rotation relatively to the remainder thereof, and means for rotating said rotatably mounted elements.

26. In combination, a light source, means associated therewith for substantially collimating a beam of light emanating from said source, and means for effecting a sinusoidal fluctuation in the intensity of said light beam, said last named means comprising, in combination, a multiplicity of light-polarizing elements positioned to intertion and the remainder of said elements being mounted for rotation in the opposite direction, and means for rotating said rotatably mounted elements.

28. In combination, a light source, means associated therewith for substantially collimating a beam of light emanating from said sourceyand means for effecting a sinusoidal fluctuation in the intensity of said light beam, said last named means comprising, in combination, a plurality of light-polarizing elements positioned to intercept said beam, a fractional wave retardation device positioned between said polarizing elements, at

least one of said elements being mounted for rotation with respect to the remainder t ereof, and means for rotating said rotatably mcunted elements.

29. In combination, a light source, means associated therewith for substantially collimating a beam of light emanating from said source, and means for affecting a sinusoidal fluctuation in the intensity of said light beam, said last named means comprising, in combination, a plurality of light-polarizing elements positioned to intercept said beam, at least one hall wave retardation device positioned, between said polarizing elements, at least one of said elements being mounted for rotation relatively to the remainder thereof, and means for rotating said rotatably mounted elements.

30. Apparatus for generating varying electrical waves comprising a source of light of substantially constant intensity,photoelectric means, a

se die polarization .ot. liiht therethrodgh' in ac- "eordence; with theieontinuoua rotation of said polarizing means, and mm for i -otatingat least one of said polafliiinwmeans.

31. Apparatus in moi-dance with so and wherein 9o means are rotated in the I 2,376,498 plurality o! lilht helm: means positioned ad- Meant-each to the other adopted to change-the- 1 1 82. Apbaratus in' accordance with claim 30 wherein at least a pair of saidpolai izing means are rotated in opposite directions.

- 33. Apparatus in accordance with claim. 30 wherein means are provided to inoculate thelight source.

' MARTIN GRABAU 

